BAGHDAD — Shiite cleric Muqtada al-Sadr has ordered a six-month suspension of activities by his Mahdi Army militia in order to reorganize the force, an aide said today.

The aide, Sheik Hazim al-Araji, said on Iraqi state television that the goal was to "rehabilitate" the organization, which has reportedly broken into factions, some of which the U.S. maintains are trained and supplied by Iran.

"We declare the freezing of the Mahdi Army without exception in order to rehabilitate it in a way that will safeguard its ideological image within a maximum period of six months starting from the day this statement is issued," al-Araji said, reading from a statement by al-Sadr.

The order was issued after two days of bloody clashes in the Shiite holy city of Karbala that claimed at least 52 lives. Iraqi security officials blamed Mahdi militiamen for attacking mosque guards, some of whom are linked to the rival Badr Brigade militia.

A spokesman for al-Sadr, Ahmed al-Shaibani, denied the Mahdi Army was involved in the Karbala fighting. Al-Sadr called for an independent inquiry into the clashes and urged his supporters to cooperate with the authorities "to calm the situation down," al-Shaibani said.

Tensions have been rising in southern Iraq as rival Shiite groups maneuver for power, especially in the oil-rich area around Basra, Iraq's second-largest city.

Al-Sadr organized the Mahdi Army shortly after the U.S.-led invasion in 2003. Since then the Mahdi Army has become the most active and feared armed Shiite group, blamed by the U.S. for driving thousands of Sunnis from their homes in retaliation for Sunni extremist attacks on Shiite civilians.

The Mahdi Army launched two major uprisings against U.S. and coalition forces in 2004. Since then, the Americans have differentiated between the mainstream Sadrist organization and what they term "rogue" elements within the force that have staged numerous deadly attacks against U.S. forces in Baghdad and elsewhere.

Authorities in Karbala locked down access to the city of Karbala today after the fierce clashes between the rival Shiite militias that forced an end to a massive religious festival.

Security was heightened in other Shiite areas to prevent clashes from spreading.

Following two days of clashes, Prime Minister Nouri al-Maliki, a Shiite, rushed to Karbala to meet with local officials trying to restore order and move the hordes of pilgrims who had descended on the city for the festival.

The Karbala office of al-Maliki's Dawa Party was firebombed during the melee.

Sporadic gunbattles raged today near two shrines protected by the Badr Brigade, the armed wing of the Supreme Islamic Iraqi Council, although violence was tapering off.

Clashes began late Monday but escalated dramatically the following day when gunmen believed from the Mahdi Army began firing on security forces and the Badr guards, according to security officials.

The fighting forced authorities to cut short the annual Shabaniya festival, which drew an estimated 1 million people from across the Shiite world.

Despite an order to clear the city center, an Al-Arabiya television correspondent on the scene reported there remained an "intensive deployment" of Mahdi Army men, waving guns in the air.

A Sadrist member of the Karbala Provincial Council, Hamed Kanoush, was detained by Iraqi security forces and members of al-Sadr's movement threatened to attack the governor's office if he was not released, according to another councilman, speaking on condition of anonymity out of security concerns.

At least 52 people were killed and 300 others injured, according to the director general of the health department in nearby Najaf who spoke on condition of anonymity. Sixty wounded people were brought to a hospital in Najaf, 45 miles southeast of Karbala, because the Karbala hospitals couldn't handle the volume of wounded, he said.

A city council member in Karbala, however, reported 38 dead and 231 injured in the fighting.

The Defense Ministry said al-Maliki had ordered the dismissal of the top army commander in the area — Maj. Gen. Salih Khazaal al-Maliki — and an investigation into his conduct.

Al-Maliki's office said security forces had sealed the city off, allowing only residents to enter, in another effort to restore order.

The clashes appeared to be part of a power struggle among Shiite groups in the sect's southern Iraqi heartland, which includes the bulk of the country's vast oil wealth.

Gunfights also broke out Tuesday between Mahdi militiamen and followers of the Supreme Council in at least two Shiite neighborhoods of Baghdad and in Kut, about 100 miles southeast of the capital, police said.

Today, authorities imposed a curfew on the Shiite city of Hilla. Security forces also sealed off several Shiite areas of Baghdad.

Elsewhere, an American soldier died today from wounds suffered the day before in fighting near the northern city of Kirkuk, the U.S. military announced.

The trouble started in Karbala late Monday as tens of thousands of Shiites were streaming into the city for Shabaniyah, marking the birth of Mohammed al-Mahdi, the 12th Shiite imam who disappeared in the 9th century. Devout Shiites believe he will return to Earth to restore peace and harmony.

Scuffles broke out between police and pilgrims as the crowd tried to push through the security checkpoints near the Imam al-Hussein mosque, the focal point of the celebrations. At least five people were killed, police said.

Early Tuesday, crowds of angry pilgrims chanting religious slogans surged through the streets, attacking police and mosque guards, witnesses said. Two ambulances were set ablaze, sending a huge column of black smoke over the city.

Some rounds struck fuel tanks on the roofs of three small hotels, setting them ablaze, police said.

With the situation spiraling out of control, police ordered pilgrims out of the center of the city, effectively canceling the celebrations which were to reach their climax Tuesday night and Wednesday morning.

Meanwhile, the U.S. military said U.S. and Iraqi special forces had captured a suspected commander of a rogue element of the Mahdi Army that targets Iraqi citizens for kidnappings and killings.

The man, whose name was not released, was picked up on Monday in Baghdad and is also suspected of attacks targeting Iraqi and U.S. forces, the military said in a statement.

Elsewhere, U.S. forces killed two terrorist suspects and detained 22 others in several raids around the country. The two were killed in an area south of Baghdad in an operation targeting al-Qaida in Iraq leaders.

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Associated Press reporters in Karbala contributed to this report but their names were withheld for their safety.